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TORONTO, Aug. 14 (UPI) - Ivan Lendl, in a fashion that threatens to become routine, defeated John McEnroe, 6-4, 6-4, today and advanced to the final of the Canadian Open tennis championship against Vitas Gerulaitis.

Gerulaitis also gained the final when Jimmy Connors, his semifinal opponent, withdrew before their match because of a back injury. ''I can think of 15 excuses, but right now I have no claim to being the No.1 player in the world,'' said McEnroe after losing to Lendl for the fifth straight time in a period that goes back to the 1980 United States Open.

Lendl does not bother to fathom the thinking of the Associated Tennis Professionals' computer, which has him ranked No.3, behind Connors and the top-ranked McEnroe.

''I would like to change the ranking, but I know that this win will not change it,'' he said. In the five victories over McEnroe, Lendl has lost only one set. Lendl's blazing serve and forehand have earned him more than $1.6 million this year. He will be seeking another $48,000 and his third consecutive Canadian Open title Sunday.

Lendl, a tall right-hander with exceptional mobility, broke his opponent's service three times. He made the first break in the seventh game and another in ninth game after McEnroe recorded his only break. Lendl's third break came in the ninth game of the final set on a clean cross-court forehand that gave him a 5-4 lead and allowed him to serve and take the match.

''It's like 50-50 whether it goes in,'' McEnroe said of his service. ''I had no consistency. My volley is not as solid. I have started staying back more, and I think that might have hurt my attacking mentality.''

McEnroe fought off double-break point and three deuces in the first game and went to deuce to hold in the third game. Then Lendl broke through after going up on triple-break point in the seventh game when McEnroe chipped a forehand long. McEnroe Forced to Scramble

McEnroe broke back in the eighth game of the first set and did not appear unnerved by a Lendl forehand that hit him in the stomach. But Lendl pounded back, converting double-break point in the ninth game on a blazing backhand that took the corner. Lendl then served out the set, allowing McEnroe only two points.

McEnroe never came close to breaking Lendl in the second set but was repeatedly scrambling to defend his own serve. Lendl broke him at love in the ninth game and, in a run of 11 straight points, was up triple-match point in the 10th game before McEnroe made the score 40-30.

McEnroe and Lendl faced each other again today, this time in the doubles semifinal. McEnroe, teamed with Peter Fleming to defeat Lendl and Josef Brabenec Jr., 6-7, 6-1, 6-3.

Lendl said he set out last winter, with deliberation, to gain an edge over McEnroe. The groundwork began with continuous practice designed to prepare him for play against left-handers.

''Before, I could not even touch his serve,'' Lendl said. ''I just lost control. But in the winter I practiced two and three hours a day with a few left-handers who would serve and volley against me. Loss of Connors a Blow

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''I have learned that a good return is the key in how to beat him. I used to watch tapes of our matches, but I learned that you cannot read his serve from his body because he turns his back to you. You have to watch the ball.''

Tournament officials were surprised by the withdrawal of Connors, who had advanced in straight sets through his first four matches. ''Connors arrived here at the beginning of the week with a strep throat and a sore back,'' a statement from tournament officials said. ''He was taking antibiotics for the throat condition. He aggravated his back further Friday in his match with Lloyd Bourne.''

John Beddington, the tourament director, said the back injury was a chronic lumbar condition from which Connors has suffered in the past.

Mrs. Lloyd, seeded first, will meet the winner of the semifinal between Susan Muscarin and Dana Gilbert. Mrs. Lloyd moved in front from the start with hard returns down the sidelines that enabled her to capture the first game. She never yielded the lead.

Miss Piatek, seeded fourth, turned in her strongest effort in the second set when she won two straight games and cut Mrs. Lloyd's lead to 3-2. But Mrs. Lloyd took the next three games and the match.

''I was very pleased with the way I played,'' Mrs. Lloyd said. ''When I play somebody, I play as well as I have to. I never really felt threatened at any point.''

Taroczy advanced with Roscoe Tanner, also unseeded, who scored a 3-6, 6-4, 7-6 victory over Kim Warwick. Taroczy took control early in the opening set, breaking Vilas in the fifth game. In the second set, Vilas was up by two break points at 4-1 and serving when Taroczy came on. He broke Vilas three consecutive times and took the last five games for the victory.

---- Mayer-Van't Hof Final

CLEVELAND, Aug. 14 (AP) - Top-seeded Sandy Mayer will face Robert Van't Hof, who was seeded eighth, in the final of the $75,000 Western Classic on Sunday.